The invention relates to apparatus for heat treatment of fine-grained material, such apparatus including a multi-stage cyclone preheater for preheating the material, a rotary kiln for final burning of the material, and a calcination zone arranged in a gas pipe between the rotary kiln and the cyclone preheater for further heating of the preheated material by means of additional fuel before the material enters the rotary kiln. The gas pipe leading from the calcination zone to the lowest stage of the cyclone preheater has an upwardly extending pipe section, a deflector section, and a downwardly extending pipe section which communicates with the lowest cyclone stage.
Apparatus of the aforesaid type already is in use. In operation of such apparatus it is not uncommon for material deposits to be formed in the gas pipe between the rotary kiln and the lowest stage of the cyclone preheater, that is to say in the calcination zone. These deposits may be partially broken up and some parts thereof carried by the stream of gas into the lowest cyclone stage of the preheater. The material deposits in this lowest cyclone stage frequently block the material outlets of the cyclone in an undesirable manner and this results in complicated cleaning operations which necessitate periods of idleness (stoppages of production).